The present invention relates to the field of electronic circuit components. More specifically, the invention relates to a method for calibrating individual potentiometers.
Potentiometers are often used to vary the electrical resistance at a particular point in an electric circuit over a range determined by the physical characteristics of the potentiometer itself. Typically, potentiometers are made from an electrically resistive material generally having a connection point at each end, thereby providing a fixed electrical resistance between the two ends. A xe2x80x9cwiperxe2x80x9d is associated with the potentiometer and is slidably movable between the two ends of the resistive material. The wiper includes a connection point that provides a selectively variable resistance range from approximately the minimum resistance of the potentiometer to the full resistance of the potentiometer. Generally, potentiometers are designated by the maximum possible electrical resistance of the potentiometer, such as 1 K, 5 K, 10 K, 100 K ohms, etc.
In addition to the maximum electrical resistance, potentiometers are further designated by tolerance ranges such as +/xe2x88x9220%, +/xe2x88x9210%, +/xe2x88x925% or +/xe2x88x921%. As is the case with almost any electrical component, it is generally less expensive to produce a potentiometer having a +/xe2x88x9220% tolerance than to produce a potentiometer having a +/xe2x88x925% tolerance. In this respect, there is often a large trade-off between the accuracy of the potentiometer and the cost of the potentiometer.
Additionally, it is also common for the linearity of the potentiometer resistance to vary over the length of the resistive material used in the potentiometer, such that equal movements of the wiper do not always produce equal changes in the resistance. Thus, each potentiometer, regardless of its manufacturing technique, stated tolerance, and quality control, has individual characteristics with respect to its maximum electrical resistance, linearity, and the length of travel of the wiper between its maximum and minimum resistance.
Potentiometers are often used to provide a variable electrical resistance that is critical to the proper operation of the electrical circuit within which it is installed. In these types of applications, the potentiometer is generally provided with a pointer or indicator that provides an indication of the relative position of the wiper with respect to the resistive material of the potentiometer. The indicator interfaces with printed indicia in the form of numbers, letters or graduation marks on a surface in fixed relationship with the potentiometer. The interaction between the indicia and the potentiometer indicator provides a relative indication of the electrical resistance at the wiper connection and thereby the resistance of the potentiometer. Typically, when a potentiometer is manufactured, the indicia are printed on the potentiometer without any testing or measurement of the actual resistive characteristics of the potentiometer.
When a potentiometer is connected in an electrical circuit, changing the potentiometer resistance can have a large effect on the output or function of the circuit in which it is installed. Therefore, in a situation where the electrical resistance of the potentiometer is critical to the performance of the circuit, the interface relationship between the indicator on the potentiometer and the printed indicia must accurately indicate the true electrical resistance of the potentiometer. In applications where relatively small changes in the resistance of the potentiometer have larger effects on the circuit itself, further calibration of the potentiometer is often required.
In this regard, the Rogers et al. U.S. Pat. No. 5,565,785 provides a method for calibrating a potentiometer that provides indicia reflecting a relatively accurate indication of the electrical resistance throughout the resistance range of the particular potentiometer. The ""785 patent describes measuring the maximum resistance of the potentiometer, a first resistance at a predetermined location, and a second resistance at some known angle from the first resistance. Based on these three measured resistances and the known angle between two of the resistance values, a series of preset index numbers are used to determine the spacing between the indicia printed on the potentiometer face. In this manner, simple and quick electrical measurements of resistance values and one angle measurement can be used to calibrate the potentiometer.
Although the above-identified method of calibrating a potentiometer somewhat tailors the printed indicia according to the resistance value of the individual potentiometer, the limited number of measurements made on the potentiometer do not provide enough accuracy for many uses of the potentiometer. Therefore, it is particularly desirable to develop a method for calibrating a potentiometer that more accurately determines the potentiometer""s resistance values at a larger number of angular positions of the wiper.
It is an object of the present invention to provide a method of calibrating a potentiometer that determines the electrical resistance of the potentiometer at a relatively large number of angular positions of the wiper. It is another object of the invention to provide a method of calibrating a potentiometer that compensates for the inherent hysteresis found in a potentiometer and the inherent hysteresis of the measuring device as result of the wiper flexibility when the wiper is moved in opposite directions within the potentiometer. Finally, it is an object of the invention to provide a method of calibrating a potentiometer that can quickly and accurately produce an average set of resistance values for the entire range of wiper movement within the potentiometer itself.
The present invention is a method of calibrating a potentiometer or other variable resistance device based on the physical characteristics of the individual potentiometer or device.
The method of the invention involves coupling an encoder to a drive shaft that rotates the wiper within the potentiometer, such that rotation of the wiper results in rotation of the encoder. The encoder is configured to generate an encoder pulse upon a predetermined amount of rotation of the potentiometer wiper.
The potentiometer is coupled to a measuring unit that is configured to determine the resistance of the potentiometer through a simple electronic circuit. Included in the measuring unit is a microprocessor and a data acquisition device (DAQ) such that the microprocessor can determine the resistance of the potentiometer based on a voltage value fed into the data acquisition device. In a first embodiment of the invention, the microprocessor is coupled to the encoder, such that upon receiving a pulse from the encoder, the data acquisition device is triggered to record the voltage representing the resistance of the potentiometer. After recording the voltage, the data acquisition device passes the recorded information to the microprocessor. Since the encoder generates pulses based on a predetermined amount of rotation of the potentiometer wiper, the microprocessor can relate the measured resistance to a change in the angular position of the wiper.
In a second embodiment of the invention, the data acquisition device is coupled to the encoder, such that upon receiving a pulse from the encoder, the data acquisition device records a voltage associated with the potentiometer. In this manner, the data acquisition device compiles an array of voltage values related to each of the encoder pulses. After the entire array has been compiled by the data acquisition device, the microprocessor can read the array and translate the voltage values and encoder pulses into angular positions and resistances of the potentiometer.
In order to calibrate the potentiometer, the potentiometer wiper is initially rotated to a first stop position. Since the encoder only generates pulses as it rotates, it is important that the encoder is in a known position before it is rotated so that the microprocessor can monitor the wiper position. Typically, the first stop position represents the maximum resistance of the potentiometer. When the potentiometer reaches the first stop position, the microprocessor records the position of the wiper. After recording the position of the wiper, the microprocessor operates a motor to rotate the potentiometer wiper from the first stop position to a second stop position. The first and second stop positions represent a resistance range to be calibrated, which is usually the physical movement limits of the wiper within the potentiometer, and should not change as the potentiometer is used. As the wiper moves from the first stop position to the second stop position, the data acquisition device records the voltage related to the resistance of the potentiometer upon receipt of each encoder pulse from the encoder. Based on the recorded voltages, the microprocessor is able to generate a set of forward resistance values, wherein each value is associated with an angular position of the wiper.
As the potentiometer approaches the location of the second stop position, the rotation of the wiper is stopped and then reversed to move the wiper from the second stop position to the first stop position. During this reverse movement, the data acquisition device again records the voltage related to the resistance of the potentiometer upon receiving an encoder pulse from the encoder. In this manner, the microprocessor is able to generate a set of reverse resistance values. Since the wiper within the potentiometer is relatively flexible, this flexibility can cause some of the hysteresis inherently associated with the potentiometer. In addition, as the wiper is moved in opposite directions, the coupling mechanism between the potentiometer and the drive shaft can cause the forward resistance value and the reverse resistance value to be different for the same angular position of the wiper.
After the forward and reverse resistance values have been measured, the microprocessor combines the forward and reverse resistance values to generate a set of average resistance values for each angular position of the wiper between the first and second stop positions. In this manner, the microprocessor generates an average resistance value for each position of the wiper, thereby reducing the hysteresis effect caused by the physical characteristics of the wiper within the potentiometer, the coupling apparatus, and the electrical hysteresis.
After the average resistance values have been determined, the microprocessor operates a label marking station to produce a label having indicia representing the actual angular position for selected resistance values within the individual potentiometer.
Other features and advantages of the invention may be apparent to those skilled in the art upon inspecting the following drawings and description thereof.